This invention relates to a process for making activated carbon from agricultural residues including seed hulls such as rice hulls, fescue hulls, wheat hulls, oat hulls and the like.
Disposal of agricultural residues has become costly as available landfill space is exhausted. In certain areas, such as California, for example, environmental laws may prohibit such disposal. It has therefore become increasingly important to develop alternatives to disposal in landfills, such as transformation into useful industrial products.
Certain agricultural residues have been processed into activated carbon. Activated carbon is widely used in both gas phase and liquid phase applications such as purification and separation of gases, recovery of solvents, purification of water, removal of color from water, industrial and municipal waste water treatment, and the like. Activated carbon is also used as a catalyst or a carrier for catalysts in chemical synthesis processes. Among the various materials which have been used for the manufacture of activated carbon are lignite, coal, bones, wood, peat, lignin, coconut shells, and petroleum residues.
Otowa, U.S. Pat. No. 5,064,805, disclosed a method of producing activated carbon by mixing coconut shell char with melted potassium hydroxide hydrate. The mixture was heated to a temperature of 500.degree. C. in the presence of nitrogen gas until the evolution of water vapor subsided, then to 800.degree. C. where activation was carried out for 100 minutes.
Sato, U.S. Pat. No. 4,616,001, disclosed a method of making activated carbon by first coking shells of macadamia nuts at 400.degree.-900.degree. C. and subsequently activating the resulting carbonized materials with steam at a temperature of 900.degree. C.
Gambel, U.S. Pat. No. 1,543,763, disclosed a method of producing decolorizing carbon by charring rice hulls without sufficient air contact to prevent burning up a material proportion of the carbon present, boiling in a caustic soda solution to remove silica, and then washing with carbonic acid.
Fujimori, U.S. Pat. No. 4,552,863, disclosed a process for producing granular activated carbon using wood as a carbon source. The wood is first pulverized and then pelletized. The resulting pellets are carbonized by heating to a temperature of 500.degree. to 600.degree. C. in an oxygen-free furnace. The carbonized pellets are then activated at a temperature in the range of 850.degree. to 900.degree. C. with steam.
Farris et al., U.S. Pat. No. 5,164,355, disclosed a method of making an oxygen-selective carbon molecular sieve from granular coconut shell char for air separation. The shells were crushed and sized to provide shell granules, which were then heated in a flowing stream of inert gas at an average temperature rate increase of about 2.degree. to 12.degree. C. per minute to reach a peak temperature of 775.degree. to 900.degree. C. The granules were held at the peak temperature for a period of time so that the heating and holding steps totaled up to about 8 hours to produce the granular char, which is then cooled in an inert gas atmosphere.